Link to the project: https://www.codecademy.com/courses/learn-python-3/projects/thread-shed
The solution video shows the use of two for loops in order to get two layers deep into the list.
Code used inside the solution video:
thread_sold_split =  for sale in thread_sold: for color in sale.split("&"): thread_sold_split.append(color)
thread_sold_split =  for color in thread_sold: thread_sold_split.extend(color.split("&")) or for color in thread_sold: thread_sold_split += color.split("&")
From my understanding of the differences between += and append is that if the expression on the right side of += is a list, it will add the whole value into the new variable instead of adding it individually. Using append will create a list of a list if the value is, for example, a list of string.
I don’t get why did the solution video shows that we need two for loops to do that when += or extend() works the same unless the value isn’t a list to begin with.